1. Field of the Invention
This invention relates to a method for radioimmunoassay (hereinafter RIA) of calcitonin, to a radioactive-iodine labeled tracer for use therein, and to a process for preparing the tracer.
2. Description of the Prior Art
Calcitonin, a peptide hormone, has an effect on the metabolism of calcium and human calcitonin was isolated in 1968 from the tumor tissue of a patient with medullary carcinoma of the thyroid.
The amino acid sequence of human calcitonin was determined also in 1968 as ##STR2##
An antibody to calcitonin was prepared using native calcitonin as an antigen obtained from the extract of medullary carcinoma of the thyroid as described in Clark et al., Lancet, 74 (1969) while an antibody using synthetic human calcitonin was prepared and then a radioimmunoassay method was developed as described in Frolich et al., Horm. Metab. Res., 3 297 (1971). However, some important problems remain unsolved in their radioimmunoassay.
When the extract of medullary carcinoma of the thyroid is used as the source of calcitonin, contamination by some polymers in calcitonin is observed as described in Neher et al., Nature, 220 984 (1968). Therefore, the serum level obtained by the radioimmunoassay does not necessarily indicate the true value.
On the other hand, synthetic and native calcitonin have a disulfide bond in the molecule, and the bond is subject to chemical change. It has been confirmed that the biological activity of calcitonin substantially disappeares due to the rupture of the disulfide bond.
Further, when native or synthetic calcitonin is labeled with radioactive iodine, some polymerized materials are present as contaminants in the product. Thus, actually available labeled calcitonin should be obtained using certain complicated purification procedures. Moreover, H. Tashjian et al., Endocrinol., 84 140 (1969) discloses that calcitonin labeled with radioactive iodine is unstable and tends to be inactivated during storage.